System of reception for modulated-wave telegraphy and telephony



July 17, 1928. 1,677,797

E. Y. ROBINSON SYSTEM OF RECEPTION FOR MODULATED WAVE TELEGRAPHY AND TELEPHONY a 7'4 & W@?li@ v I w -m Allan 7% July 17, 1928. 1,677,797

7 E. Y. ROBINSON SYSTEM OF RECEPTION FOR MODULATED WAVE TELEGRAPH! AND TELEPHONY s Sheets-Sheet 2 Filed March 19, 1925 Atlorlz ey.

July 17, 1928. i 1,677,797

E. Y. ROBINSON SYSTEM OF RECEPTIO/N FOR MODULATm) WAVE TELEGRAPHY AND TELEPHONY Filed March 19. 1923 3 Sheets-Sheet. 3

.Allorlfqy.

frequency of 2tf). I

is complex then, if the component:

Patented July 17, 1928;

UNITED STATES 1,677,797 PATENT OFFICE;

ERNEST YEOMAN ROBINSON, on WITHINGTON, ENGLAND.

SYSTEM OF RECEPTION FOR MODULATEDWAVE TELEGRAPHY AND TELEPHONY.

Application filed March 19, 1923, Serial K0. 626,140Qai1d in Great Britain March 28, 1922.

This invention relates to methods of. reception of modulated wave telegraphy and telephony in which a carrier wave is modulated such, for instance, as in wireless telephony or that system known as wired wireless and has for its object to provide an improved method of reception and arrangement of circuits whereby the efficiency of detection or de-modulation is increased and also whereby distortion due to interference effects of the various components of the received oscillations may be eliminated or reduced to a minimum.

In systems of communications employing a modulated carrier wave the following sequence of operations may take place:

1. The modulating wave is caused to vary the amplitude of an oscillatory current which is separately generated and which is known as a carrier wave. The carrier wave is generally so modulated that its envelope corresponds to the modulating wave. It may be resolved into a number of component wavesof different frequency. Considering first, for simplicity, the elemental case wherein a carrier wave of frequency (a) is modulated sinusoidally by a wave of frequency then waves of three frequencies are"prcsent in the resulting wave. Thus there are, firstly the waves of the original carrier wave frequency (a), secondly waves of frequency (011+ and thirdly waves of WVhen the modulating wave waves in the modulating wave vary in frequency from f, to the resulting component frequencies of the modulated waves willvary from (ad-f to (rt-t7), which range of frequencies is known as the upper side band, and (n/,) to (af which range is known as the lower side band. inaddition there are of course present waves ofcarrier wave frequency. V

2. The signals may be transmitted by means of the whole of the waves present in the modulated wave, that is, by means of the carrier wave and theupper and lower side bands, or the carrier wave and one side band. only may be transmitted, or only one or both side bands may be transmitted, the necessary carrier wave for de-modulation being supplied at the receiving station.

3. The received waves are (lo-modulated at the receiving station. For the process of ie-modulation or detection it is essential that carrier waves should be present, and as it has been hereinbefore stated, these may be transmitted to the receiving station or alternatively they may be supplied at the receiving station itself In wireless telephony it is the present practice to transmit by means of the carrier wave and both side bands. For the process of die-modulation or detection the carrier waves should preferably be of large ampli tude with respect to the waves lying in the upper or lower side bands in order to give increased efficiency of detection. Moreover,

'vices may be made for amplifying oscillations of one frequency, especially when it is not desired that the envelope of such amplified waves should correspond completely to the envelope of the original waves, whereas when amplifying devices are required for amplifying waves over a range of frequencies and when theenvelopeof the original waves must be preserved during amplification, such amplifiers are relatively much less selective Now, according to this invention, I provide means whereby the received waves of carrier wave frequency are amplified in an efficient manner by means of the former type of amplifying or relay devices referred to above, which devices may be made very responsive to the carrier waves, whilst the envelope-of the waves in the upper or lower side bands or both are preserved by employing either aperiodic circuits or oscillatory circuits having the requisite decrement, and in the case where such waves are amplified this is effected by relay or amplifying devices of the second typementioned above.

Furthermore I provide (in the case where the carrier wave amplification is effected in separate circuits as hereinafter explained) means whereby such amplified carrier waves are recombined with the side band waves before or during the de-modulation. In the preferred method the carrier wave is amplified by means of thermionic valves, the

' circuit.

coupling between the valves being effected by means of low decrement oscillatorycircuits tuned to the carrier waves; these circuits may be made of low decrement by incorporating with them de'vicesor circuit arrangements having a negative resistance effect; for instance, a valve employing a retroactive coupling to the oscillatory .circuit may be employed. By this means a very high degree of amplification of the carrier waves may be'obtained. Moreover, since the circuits involved have a very low decrement such amplified oscillations will not vary greatly in amplitude even when the varia tions in amplitude of the incoming carrier wave is considerable, because the oscillatory currents in such low decrement circuits cannot respond greatly to amplitude variations which take place at coustic. frequencies. They may be made constant by introducing a limiting device such, for instance, as a dimmed filan'lent amplifier in the amplifying In an alternative method one of the amplifying devices may have relay characteristics, that is, the output of the device is independent of the magnitude of the input to the device. The waves of side band frequencies may be amplified by nonselective amplifiers. In the case of the higher frequency waves employed in wireless transmissions such amplifiers may consist of valve amplifiers, the coupling being of the tuned circuit type; at the lower frequencies which are en'iployed such as in wired wireless such amplifiers will be too selective and amplifiers giving band frequency characteristics such for instance as a plurality of oscillatory circuits may be employed, as also resistance couplings or transformer couplings. For (lo-modulation there are there fore present oscillations of carrier Wave frequency which have a large amplitude as is requisite for efiiciency of detection or demodulation and, when the frequency of the carrier waves is l.ow,-as is requisite for distortionless den'1odul.ation there are present oscillations which have amplitude fluctua tions closely following those of the received waves as is requisite in avoiding or reducing distortion. By this invention the amplitude of the carrier waves present in the oscillations applied to the de-modulating valve may be made so 'reat that (ls-modulation is entirely due to rectification on the straight portion of the anode characteristic thus giving the maximum posslble efficiency of demodulation.

amplified by means of an amplifier 4 responsive only to the said waves. The amplified carrier wavesare recombined with-the other waves before or during the process of dc-modulation" which i effected at 3, the

at 6. It is coupled to the tuned oscillatory.

circuit 7 and the received oscillations are amphfied 1n the usual way by meansof the amplifier comprising the valve 8 and the' transformer 13. .The carrier Waves are amplified separately by valve 9, the output circuit of this valve being causedto-excit'e the low decrement circuit 10. In the example shown the resistance of'the circuit 10 is compensated for by the introduction of negative resistance provided by means of a valve 11 and back coupling 12. Oscillations in the circuit 10 are thus substantially'at constant amplitude owing to the extremely low decrement of this circuit. The oscillations in circuit 10 are caused to react on the secondary of the transformer 13 and induce therein oscillations of carrier Wave frequency which, combined with the amplified sidebands of the received oscillations, are (ls-modulated in a valve 14-. In the example shown the frequency of the carrier wave is high (corresponding, for example to a wave length of 1000 metres) and for this frequency the grid leak method of detection is most suitable. However, any method of detection may be employed. The valve 11 may be of the dimmed filament type in order to maintain the oscillations in circuit 10 more nearly constant. In such a case it should preferably be arranged. thatthe operating poten tial of the grid is such that the valve operates at the point of inflection of its anode characteristic.

Fig. 3 illustrates another method of ap plying this invention. The received oscillations flow through the loading coil 15. The

carrier waves are amplified by means of the circuits and valves shown at 16, 17, 18 andv 19. The circuit 16 consists of a secondary oscillatory circuit coupled to the antenna loading coil-15. The received carrier waves are amplified in the valve 17 and areinduced in the oscillatory circuit 18, which is given a low decrement by means of a back coupling from the valve 19. The side band oscillations are amplified by means of the valve 20 and. the amplifier is of the resistance coupled type, the resistance being shown at 21. The carrier wave oscillations arecoupled back to the side band oscillations by means of the condenser 22 and detection is effected in the usual manner by means of the valve 23. Owing to the fact that it is desirable that the decrements of the oscillatory circuits in the amplifier which is work'- ing at the side band frequencies should be high, there is little to be gained in using a secondary oscillatory circuit coupled to the antenna and hence it is preferable that the first valve of the said amplifier should be connected directly to the aerial in order to provide as large an amount of power as possible. The circuit 16 may be given a low decrement if desired by means of a back coupling from the valve 17.

Fig. 4 shows an application of this invention to wireless receiving apparatus when the frequency of the carrier waves is comparatively low. Owing to the fact that at such relatively low carrier wave frequencies the variation of side band frequencies represents an appreciable proportion of the carrier wave frequency, it is necessary that where tuned circuits are employed such circuits should have band characteristics, that is, that these circuits should present substantially the same impedance to all oscillations of the side band frequencies which are being amplified or otherwise dealt with. This may be effected by employing circuits of the 'so-called filter type, such as, for instance, a plurality of oscillatory circuits, each tuned to a slightly different wave length. In the example shown the side band oscillations are received by the antenna which is given band characteristics by means of the several oscillatory circuits at 24. These oscillations are amplified as shown by means of the trans former coupled amplifiers. The carrier wave oscillations are received at a separate antenna shown at 25 and are separately amplified. The decrement of the antenna 25 may be reduced by means of the back coupling 26 from the first valve of this amplifier. The carrier wave oscillations are caused to excite the low decrement circuit 27. They are coupled with the side band oscillations by means of the coil 28 which may be inserted in the plate circuit of the last valve of the carrier wave amplifier. This coil is coupled magnetically to the secondary of the transformer 29. The combined oscillations are demodulated by means of the valve 30. Alternatively the amplified carrier waves may be introduced into the side band wave amplifier at an earlier stage and such an arrangement would result in an increased efficiency of detection. However, owing to the coupling back through the valves due to intervalve ca pacity and due to the coupling betweenthe antenna: such an arrangement would be liable to oscillate easily, and to reduce the tendency to oscillation an appropriate number of amplifying valves may be inserted between the antenna and the point at which the amplified carrier waves are introduced. Similarly, in all other receiving arrangements there is a great tendency to oscillate this tendency may be reduced by increasing the number of valves in the side band wave amplifier prior to the introduction of the amplified carrier waves.

Fig. 5 shows an application of this invention to receiving systems for high frequency signalling currents which are transmitted along wires. The received oscillations enter The carrier waves are amplified separately by means of the valves 40 and 43 and the low decrement circuit 41. The circuit 41 is given a low decrement. by means of a back coupling from the plate circuit of valve 43. The oscillations are file-modulated by means of valves operating on the so-called push pull principle. These valves are shown at 34 and 35. The side band oscillations are transferred to the grids of these valves by means ofthe transformer formed by the two windings 32 and 33. The amplified carrier waves are introduced into this circuit by means of the transformer formed by the two windings 38 and 39, the winding 39 being connected in the plate circuit of valve 43. After ole-modulation the valve current is transferred to circuit 37 by means of transformer 36.

To effect an economy of valves the individual amplification of the carrier waves and side band waves may be effected by one set of valves, the coupling circuits being such that each is composed of a plurality of circuits so designed that the carrier waves are amplified to a larger degree than the side band waves, and that the carrier waves are amplified in low decrement eircuitswhilst the side band waves are amplified in circuits which preserve the envelope of such waves. F 1g. 6 shows a method of separately amplifylng the carrier waves and waves of side band frequencies while amplification is effected by means of valves common to both circuits. The example is shown applied to the reception of wireless telephony. The received oscillations pass through the loading coil 44. This is coupled to the multiplicity of coils shown at 47 which provide a device having band characteristics and responding to th-e'side band frequencies. waves are induced mainly in the oscillatory circuit 45 which is given a low decrement by means of the retroactive coupling provided by the coil 4 This coupling should be so arranged t-ha it does nothave appreciable effect in the bandfilter 47 or the antenna loading coil 44. To further reduce the possibility of negative resistance effect being introduced into that portion of the circuits which responds to the side band oscillationslin the case shown this is the band filter 47) it may alternatively be arranged that the resistance of the oscillatory circuit 45 is reduced by means of a negative resistance device which is so coupled thereto and it is only responsive to oscillations flow- The carrier lit ill

ing round the'said circuit. This may preferably be effected by arranging a valve whose input and output circuits are coupled magnetically to the circuit 4C5 only. The other low decrement circuits may be similarly treated. The Waves of side band frequency are amplified by means of the transformercoupling. l8 while the carrier waves are amplified in the low decrement circuit ll), the said circuits being connected in series.

The plate circuit of the first valve is con nected to the oscillatory circuit 49 by means of the inductance 50 and the said oscillatory circuit is given a low decrement by means of the retroactive coupling provided by the coil 51. In the process of demodulation the side band waves pass through a transformer 52 while the carrier waves are induced in an oscillatory circuit 53 by means of the coupling provided by the coil 54:. The circuit 53 is given a low decrement by means of the valve 56 and the back coupling 55. The waves are (lo-modulated by the-two valves 57 and 58 which operate on the push pull principle. The acoustic frequency oscillations are then transferred to the telephone 60 by means of the transformer 59.

Fig. 7 illustrates an alternative method of combining the amplified carrier waves with the amplified side band waves. In the fig are the amplified circuits are similar to those already described but the de-n'iodulating valve 61 has two grids. One of the said grids is connected to the output circuit of the amplifier which amplifies the side band waves and the other grid is connected to the output circuit of the amplifier amplifying the carrier waves.

In the specification and claims the amplification of side band frequencies has been referred to. It will be understood, however, that in these cases the carrier wave will generally be amplified by the said amplifier unless the carrier wave frequency is very low, and the amplifier of side band waves is responsive only to one side, for if the amplifier is responsive to both side bands then it must also be responsive to the carrier wave since the frequency of this lies between that of the side bands. Generally, the carrier waves will be amplified in the side band amplifier, to the same degree as the side band waves themselves, but such amplification of the carrier-wave will. be small compared to the amplification which is obtained by means of the amplifier which selectively amplifies the said carrier wave.

It is desirable that when the carrier waves are recombined with the side band waves they should have the correct phase relation thereto. They should preferably have the same phase relation as if they had not been amplified independently. This may be arranged by suitably proportioning the constants of the circuits and by reversing the direction of the coupling coil when necessary.

By constants of the circuits is meant such properties as the inductance, the capacity .and the coupling, and such like of the circuits. It is impossible to say exactly how this may be attained, since it will depend to a large degree on such factors as intervalve capacities and the self capacities of the inductive coil windings employed in the amplilicr circuits. The phase of the carrier wave oscillations may be adjusted in particular cases by altering the ratio of the capacity and inductance in the circuits involved, and also by varying the coupling between two or more such circuits. For example, the phase of the oscillations in the coupling coil 28 (see Fig. 4) may be varied by the condenser 28 (shown in dotted lines).

The decrement of the low decrement cir cuits should preferably be made as lowas possible. The circuits may be arranged so that they are on the fringeof self-cs 'llations or they may be oscillating feebly so that the resultant oscillations formed are controlled entirely by the incoming waves. In the latter case the carrier waves control and lock into phase oscillations which are, to all intents and purposes, separately generated; by this means very strong oscillations of carrier wave frequency may be obtained. It thus highly desirable that the carrier wave frequency of the transmitting station should be constant. In order that the circuits may be very sensitive it is necessary that they should have no back lash effect. This may be accomplished by ar ranging that the grid potential of the valve concerned is at the point of inflection of the valve characteristics, (or the straight portion of the characteristic when there is no well defined point of inflection). This may be accomplished by means of a grid potentiometer and for these low decrement circuits potentiometers are essential if the circuits are not otherwise suitably designed.

[is it has been hereinbefore stated the frequency of transmission should be main- When this IS not POSSllJlG' tained constant. and the carrier wave varies slightly with the degree of modulation as, for instance,

with a resistance modulated antenna current peak. For example, with a resistance moduunderstood that this invention is not limited to the circuits herein described.

I claim as my invention i 1. In a communication system in Which a carrier Wave of constant frequency is partially modulated by audio frequency currents and the original full carrier Wave and at least one ofthe side band components of the resulting complex oscillations transmitted, the method of reducing distortion in and increasing the efliciency of the demodulation at the receiver, Which comprises receiving the original wave components as transmitted, increasing the amplitude of the transmitted carrier Wave component relative to that of a. side band component While maintaining substantially constant the amplitude relationship of the various frequencies of said side band component, and finally effecting de-modulation of the oscillations thus modified.

2. In a communication system in which acarrier Wave of constant frequency is partially modulated by audio frequency currents and the original full carrier Wave andatleast one of the side band components of the resulting complex oscillations transmitted,.the method of reducing distortion in and increasing the eiiiciency of the demodulation at the receiver which consists in receiving the original Wave components as transmitted, increasingthe amplitude of the transmitted carrier Wave component relative to that of a side band component, maintaining the increased. amplitude of the carrier Wave component substantially constant While maintaining substantially constant the amplitude relationship of the various frequencies of said side band component, and finally effecting de-n'iodulation of the oscillations thus modified.

3. In a communication system in which a ca rier Wave of constant frequency is partially modulated by audio frequency currents and the original full carrier Wave and at least one of the side band components of the resulting complex oscillations transmitted, the method of reducing'distortion in and increasing the efiiciency of the demodulation at the receiver which comprises receiving the original Wave components as transmitted, increasing the amplitude of the transmitted carrier Wave component relative to that of a side band component by generating feebleoscillations Which are controlled by the received carrier Wave-and are caused to look into phase therewith While maintaining substantially constant the amplitude relationship of the various frequencies of said side band component, and finally effecting de-modulation of the oscillations thus modified.

4. A receiving system for modulated high frequency electric oscillations comprising means for receiving the original modulated carrier Wave as transmitted, means for amplifying the carrier Wave component of the received oscillations relative to the side band components, While maintainingsubstantially constant the amplitude relationship of the various frequencies of the side band components, and means for finally de-modulating or detecting the modified oscillations.

5; A receiving system for'modulated high.

frequency electric oscillations comprislng means for receiving the original modulated carrier Wave as transmitted, means including low decrement oscillatory circuits for amplifying the carrier Wave component of the received oscillation-s relative. to the side band components While maintaining substam tially constant the amplitude relationship of the various frequencies of the side band components, and means for finally demodulating or detecting the oscillations thus modi tied. I

6. A receiving system for modulated high frequency electric oscillations comprising means for receiving the original modulated carrier Wave as transmitted, means includ-. ing low decrement oscillatory circuits adapted to oscillate feebly so that'the resultant oscillations are controlled by the received carrier Wave and the resulting oscillations Will look into phase therewith. for amplifying the carrier Wave component of the received oscillations relative to the side band 'wave components While maintaining substantially constant the amplitude relationship of the various frequencies of the side band components, and means for finally demodulating or detecting the oscillations thus modified.

In testimony whereof I have hereunto subscribed my name this 5th day of March,

ERNEST YEOMAN ROBINSON. 

